The dust generated by the use of power tools is of a fine particulate nature and floats easily in the surrounding air under normal operating conditions. Vibrations associated with the operation of the power tools also tend to disperse this dust still further. A working environment containing suspended dust is unpleasant, inconvenient and unhealthy for users of these tools. Moreover, once the dust has settled, a user is required to clean-up and remove the scattered dust from the environment. In view of these problems, a number of prior art arrangements have already been developed for extracting dust from around the operating region of power tools.
Early ideas for dust extraction systems are discussed in two sections headed “Dust Extraction Systems” and “Dust Separation Units” on pages 104-116 of the book “Ergonomic Tools in our Time”, an Atlas-Copco publication, by Bo Lindqvist et al., 1986 (ISBN: 91 7810 535 8). FIG. 1 illustrates the dust extraction broken down into a chain of general components. A dust extraction hood 200, located close to the point where the dust is generated 100, is operably connected to a suction hose 300. The suction hose 300 is in turn connected to suitable pipework 500 in fluid communication with the suction hose 300 with some separation means 600 for separating the dust from air taken in by via a vacuum source 700 that generates a flow of air through the dust extraction system. A swing arm 400 supports the suction hose 300. There are several examples of means for separating the dust from air taken in by the dust extraction system, including cyclones and filters. Examples of the vacuum source for generating a flow of air through the dust extraction system include fans and root pumps. However, it can be seen in this book that although the power tools discussed are hand-held tools, the dust extraction system which is intended for use in association with them is a much larger, facilitating a more permanent mounted system.
Examples of more recent ideas for dust extraction techniques for use with hand-holdable power tools which are highly portable concentrate on the idea of using the same motor which provides power to the tool for its main function as also being used as the source of power for generating a flow of air through the dust extraction system. Contrary to the system shown in FIG. 1, this arrangement allows a considerable saving on space and weight. Since the motor which provides power to the tool's main function also acts as the source of power for generating a flow of air through the dust extraction system, there is no need to locate the means for separating the dust from air taken in by the dust extraction system at a distance which is remote from the region where the dust is generated, namely the point of contact between a working element (e.g., a drill bit, saw blade, sheet of sandpaper) which is held by a working head of the power tool and a workpiece, such as a piece of wood. Consequently, there is also no need for a suction hose or other pipework to connect the region where dust is generated with the means for separating the dust from air taken in by the dust extraction system. Instead, a short duct built into the power tool itself will generally suffice. Accordingly, these dust extraction systems generally comprise; a duct having an inlet located proximate the point of contact between the working element held by the power tool and a workpiece; a dust collection chamber having an inlet in fluid communication with an outlet from the duct; an outlet of the dust collection chamber in fluid communication with atmospheric air; a fan operable to transport air from the inlet of the duct to the outlet thereof; and, a filter situated between the outlet of the dust collection chamber and atmospheric air.
An example of such a dust extraction system is described in U.S. Pat. No. 6,514,131. This patent shows a sander having a sanding plate for holding a sheet of sandpaper, a fan for transporting air from the point of contact between the sandpaper and a workpiece through a duct to an outlet thereof, which exhausts into a dust collection chamber. The dust collection chamber is a rigid, self-supporting box having a cover with a plurality of openings in fluid communication with atmospheric air. A filter element that extracts dust from the air passing is positioned between openings and the atmospheric air is a filter element which extracts dust from the air passing to atmosphere, causing it to be held in the dust collection chamber.
A second example of such a dust extraction system is described in U.S. Pat. No. 6,224,471. This patent also shows a sander, which has a platten for holding a sheet of sandpaper, a fan with blades for transporting air from the point of contact between the sandpaper and a workpiece through a duct into a dust collection chamber. In this case, the dust collection chamber is formed from a porous plastic material, so that the interior of the dust collection chamber is in fluid communication with atmospheric air and the body of the chamber itself acts as a filter for extracting dust which is collected therein. A membrane formed across the inlet to the dust collection chamber acts as a trapdoor to prevent dust returning down duct under the action of gravity when the fan is switched off. In U.S. Pat. No. 6,224,471, the dust collection chamber is substantially cylindrical in shape and therefore has a substantially circular cross-section, and the inlet thereto is located on one of the end faces of the cylinder, with the curved surface of the cylinder and the other end face thereof acting as the outlet. Moreover, U.S. Pat. No. 6,224,471 teaches that the dust collection chamber is substantially cylindrical in shape and therefore has a substantially circular cross-section, the inlet thereto is located on one of the end faces of the cylinder, with the curved surface of the cylinder and the other end face thereof acting as the outlet.
A further example is described in U.S. Pat. No. 4,967,516. This patent also describes a sander having a platten for holding a sheet of sandpaper, a fan for transporting air from the point of contact between the sandpaper and a workpiece through a duct having an outlet which exhausts into a dust collection chamber. In this case, the dust collection chamber is a semi-rigid frame having a plurality of windows formed therein in fluid communication with atmospheric air. Each of the windows is covered by a mesh fabric which extracts dust from the air passing to atmosphere, causing it to be held in the dust collection chamber.
A final example of a known dust extraction system for use which a power tool is shown in FIG. 2. Here, a drill 10, having a working head (chuck) 12 for holding a drill bit, is provided with a dust collection chamber 14, which is connected via a duct 16 with a fan housed in drill 12. The duct 16 is provided with an inlet 18 locatable in proximity to a point of contact between the drill held by chuck 12 and a workpiece. The dust collection chamber 14 contains a fabric bag 20 as shown in FIG. 3. The fabric bag 20 comprises a plastic frame 22, which includes a centrally located tube 24 open at both ends and having a mouth 26, as shown in FIG. 4, where the frame 22 has been removed from bag 20 to show its construction. The inlet 18 to duct 16 is so arranged that during operation of the fan, dust-laden air passes through the mouth 26 of tube 24 in the direction indicated by arrow A in FIG. 4 into the interior of bag 20 in the direction indicated by arrow B in FIG. 4 where the dust is collected. Filtered air then passes outwards through the bag 20 into dust collection chamber 14 and thence down duct 16 to the fan housed in drill 12, where it is exhausted to atmosphere through the vents 28 shown in FIG. 2. Thus, this prior art dust extraction system is distinguished from those disclosed in U.S. Pat. Nos. 6,514,131, 6,224,471 and 4,967,516 in that it uses a “clean fan”, rather than “dirty fan” dust extraction technique. In other words, in this system, the air transported by the fan is filtered before it makes contact with the fan, rather than after. In this respect, the dust extraction system described with reference to FIGS. 2 to 4 is analogous in both the layout of its components and its mode of operation to a conventional domestic vacuum cleaner.
However, all of the prior art dust collection systems shown in U.S. Pat. Nos. 6,514,131, 6,224,471 and 4,967,516 and in FIGS. 2 to 4 below suffer from the disadvantage that the filters used therein easily become blocked with dust, thereby rapidly diminishing the performance of these dust extraction systems. Moreover, unlike the dust extraction system discussed above in relation to FIG. 1, since the motor which powers the fan for transporting air through these dust extraction systems also provides motive power to the working head of the power tool, if the filters therein do become clogged with dust, the operation of the fan cannot be increased by a similar amount to compensate.
Accordingly, the present invention seeks to provide a solution to this and other technical problems with the types of dust extraction systems described in U.S. Pat. Nos. 6,514,131, 6,224,471 and 4,967,516 and in relation to FIGS. 2 to 4 below, without engendering the problems with the alternative dust extraction system described above in relation to FIG. 1.